Modern-day diagnosis and treatment of patients undergoing surgery and other critically ill patients often require the measurement of blood gases. Blood gas measurement is a term that has come to stand for the measurement of pCO.sub.2, pO.sub.2 and pH. These blood parameters are regarded as critical measures in the clinical assessment of pulmonary and cardiovascular function.
Current hospital practice for measuring blood gases is both time-consuming and expensive. A sample of the patient's blood must be specially prepared and treated before it is sent to a dedicated blood gas analysis instrument within the hospital laboratory. While this instrument performs the blood analysis quickly, significant time delays occur in taking the sample to the analysis instrument and returning with the analysis results. Although some of the time lost by processing the blood samples at a remote location has been regained by placing blood gas analyzer instruments in or near surgical and other critical care units, the instruments are expensive and require the assignment of specially trained laboratory personnel.
A variety of methods for making bedside blood gas measurements have been proposed. The transcutaneous method, which makes blood gas measurements through the skin, is not accurate when used with adult patients, and it does not measure pH. Direct contact measurements, made by sensors placed within the vascular system or by bringing blood to sensors outside the patient's body, have also been proposed. Miniature electrochemical sensors have proven to be unstable and expensive. Miniature field-effect transistors (FETs), controlled by chemicals or ions, continue to present various developmental problems. Miniature gas chromatographs or mass spectrometers are expensive to build, service, and maintain. Some miniature optical-based systems rely on fluorescence. These systems interrogate a fluorescent dye with light at a first wavelength. The dye then emits light at a second wavelength or, in some cases, light at second and third wavelengths. Hydronium ions in a sample diffuse into the dye so that the pH of the dye becomes equalized to the pH of the sample. The intensity of the light emitted by the dye is a function of the pH of the dye. The intensity of the light emitted by the dye is thus an indication of the pH of the sample. The interrogating light for these conventional fluorescent systems must be at a short wavelength which is incompatible with solid-state light sources. Furthermore, fluorescence pH sensors are inherently inefficient since optical systems are generally capable of capturing only a small portion of the light emitted by the fluorescent dye. Fluorescence pH sensors also require optical fibers that are capable of operating over a relatively wide bandwidth. However, suitable optical fibers are difficult to produce, and they have other characteristics that limit performance. Other miniature optical systems use electromagnetic energy at other short wavelengths. These systems are not compatible with solid-state hardware.
It would be particularly useful then, to have an electro-optical pH and gas sensor that is based on changes in the absorption/transmission of specific chemical indicators that operate at longer wavelengths and that can be supported by cost-effective, solid-state electronics. Additionally, it would be useful to use such sensors for blood pH and gas measurements and to make such sensors so that they are inexpensive and thus disposable after use with a single patient.